Astronomy reports:

In the 2013 movie Gravity, Russia destroys its own spy satellite with a missile, creating a cloud of shrapnel and subsequent orbital collisions that rip apart spaceships and astronauts alike. Earth orbit is a place where life imitates art: Eight years later, Moscow launched an actual missile strike on a defunct Soviet satellite, blasting thousands of pieces of metal through space and threatening the International Space Station (ISS) at the nearby altitude of 250 miles (400 kilometers). As the lethal grapeshot sped through space 10 times faster than a rifle bullet, astronauts and cosmonauts huddled in escape pods. Luckily, the debris overshot them, avoiding a real-life Gravity sequel.

This close call was not a lone event. In 2009, history’s first collision between two intact satellites — an American Iridium 33 and a defunct Russian Cosmos-2251 — generated at least 2,000 pieces of debris, some of which passed on uncertain trajectories about 7 miles (11.3 km) from the ISS in March 2012 as crew sheltered in two docked Soyuz craft. And just last year, Chinese crew aboard the Tiangong space station delayed their return to Earth and left their replacements temporarily without an evacuation vehicle when a piece of space junk smaller than 0.04 inch (1 millimeter) hit their docked return capsule. Fallout from these incidents remains in orbit today, along with myriad debris from other adventures. These fragments are collectively known as space junk.

Scientists warn that space junk may be fast approaching a critical threshold that sparks cascading satellite collisions, creating an impassable belt of debris and rendering that region of Earth orbit too dangerous to use. This is called Kessler syndrome, a concept outlined in a 1978 paper by NASA researcher Donald Kessler. Surpassing that threshold is not inevitable, but avoiding it will require considerable engineering and stringent international cooperation. [Continue reading…]